<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        let str = '中国'
        let str1 = '你好'
        //   console.log(11)
        // console.log(str)
        // console.log(str1 + str)
        // console.log(str1 + str + 11)
        // let name = prompt('请输入名字')
        // let age = prompt('请输入年龄')
        //    document.write(`大家好，我叫${name}，今年${age} 岁了`)
        // let num
        // console.log(num)
        // console.log(num + 1)
        // console.log(undefined + 1)
        // console.log(null + 1)

        console.log(typeof (null + 1))
        console.log(typeof null)
        console.log(typeof undefined)
        console.log(typeof (undefined + 1))


    </script>
</body>

</html>